The present invention relates to systems and methods mounting electronic components to flexible substrates using a laser, wherein the flexible substrates have a low glass transition temperature.
Electronic components such as surface mount components may be soldered to printed circuit boards (FR4 or similar material) by reflowing solder paste. Typically a solder paste is printed on the circuit board and then the board is populated with electronic components and other devices. The populated boards are then transferred into a reflow oven and raised to a temperature high enough to liquefy the solder.
Unfortunately, this method of soldering has several drawbacks or limitations. For example, substrates that can tolerate high reflow temperatures and long durations at those temperatures may only be used in this process. For instance, flexible substrates having low glass transition temperatures would be damaged using prior art methods. However, it would be desirable to use lighter and cheaper plastic substrates such as flat flex cables especially in the automotive environment. Unfortunately, these lighter and cheaper plastic substrates and flat flex cable have a much lower glass transition temperature. Thus, if conventional methods for reflowing solder are used with these plastic substrates, the substrates will not survive the process.
Therefore, there is a need for a new and improved system and method for soldering electronic components such as surface mount components to flexible substrates. This new and improved system and method should reflow solder paste to interconnect the electronic components to circuit traces on a flexible substrate. Further, the new and improved system and method should not damage the flat plastic substrate during the reflow process.
In an aspect of the present invention a new and improved method for soldering electronic components to a flexible plastic substrate is provided. A diode laser is utilized to reflow solder paste printed on a plastic substrate. Such substrates that may be used have low glass transition temperatures, for example, PET. Beneficially, these polymer substrates absorb very little of the energy output of the diode laser.
In another aspect of the present invention, a method for using a diode laser to reflow solder on a flexible substrate is provided. This method. advantageously speeds up the soldering process. The process includes flipping the electronic components (such as surface mount components) so that a light colored side of the electronic component faces the diode laser. The laser beam may be rastered across the surface of the substrate so that laser radiation sweeps across a strip (along a common axis) on the populated substrate. In this manner, the solder paste in the laser beams path is reflowed and an electrically connection is achieved between the substrate and the electronic components. Advantageously, the laser beam does not damage the plastic substrate that is exposed to the beam since the substrate""s properties are such that minimal laser radiation is absorbed by the substrate. The rastering of the laser beam across the entire substrate surface will greatly reduce process time since the laser controller does not have to go to memory to find the next location, perform calculations, check fiducials, etc. and move to the next component.
The soldering process of the present invention can also be used to solder the surface mount components from the back (underneath) of the substrate because of the transparent nature of the plastic to the laser""s output. This will eliminate the spatial interference of the components with the laser beam when soldering from the front (top) side of the substrate. Again, the rastering method can be used in this case.